Since actual software programs implicate a continuously increasing number of functions for the user, graphical user interfaces (GUIs), such as icons, cursors, pop-up windows, and is menus are of great importance to allow a comfortable interaction of the user with the software program. Current desktop GUIs typically provide one or more drop-down or pop-up menus and a large number of icons to select specific functions. Traditional menu systems are arranged in a top-to-bottom linear configuration, where the menu items are logically arranged or similar functions are arranged groupwise. Especially, when the list of menu items gets very long, it becomes difficult for the user to quickly review the list and even more difficult to recall from prior selections the exact position of specific menu items. The drop-down menus are often nested several layers deep. Each nested layer of the menu structure is usually displayed as a separate drop-down menu in a rectangular shape that is visually connected to the previous layer of the menu structure. When choosing an option from such a drop-down menu, the cursor has to be moved over a path substantially corresponding to the horizontal or vertical lines along which the options are arranged. In the case that a graphics tablet is used as an input device to manipulate the software these precise movements for navigating through a menu structure are very difficult to fulfill even for an expert user.
To facilitate the navigation within GUIs, an alternative user-interface technique was developed, the so-called “pie” menus, which are two-dimensional and much easier to use and faster than conventional linear menus. Especially for the usage with alternative pointing devices as data-tablets or touchscreens, this special menu structure allows a quick navigation through the different menu layers. The pie menus are round menus, wherein the menu items are positioned around the center. The target regions are shaped like the slices of a pie, and the cursor starts in the center of the menu in an inactive region. The active regions are all adjacent to the cursor, but each in different directions (Don Hopkins, Dr. Dobb's Journal, December 1991). Most known pie menus only contain one menu layer with selectable menu items. Therefore, a limited number of menu functions can be accessed using such a menu structure. Alternatively, after selection of a menu item, a second menu, which can be either a linear menu or a further circular menu, appears (Kurtenbach & Buxton, Proceedings of InterCHI '93, 1993, 482-487). These structures are complicated, require wide movements of the cursor and cannot be navigated intuitively.